X-ray tube



Feb 25, 1941- J. H. vAN- DER TUUK. ETAL 2,232,831

x-'RAY TUBE- Filed Aug. 1, 193s fflYI .f1-IBI.

Patented Feb. 25, 1941 'i UNITED STATES PATENT OFFICE 2,232,831 X-nAY' TUBE and Trust Company,

trustee Hartford, Conn., as

Application Augustv 1, 1938, Serial No. 222,520 In Germany August 2, 1937 4 Claims.

Rotary anodes of X-ray tubes have to satisfy the condition that in any position of the tube it must be possible to keep them in rapid motion by means of a very slight torque. Although in itself it was obvious, in order to full this condition, to journal the anode on balls and in the course of the last twenty years several constructions having rotary anodes journalled in ball-bearings have been suggested, this journalling involves such difculties that the use thereof, at least for anodes of the heavy type, has proved in practice to be impossible. Experiments in this respect were unsuccessful because the ball-bearings, after having 'operatedfor some time, exhibited a highly increased friction with the result that the speed of rotation rapidly decreased and the tube became unt for use. The use of bearing lubrication seemed a priori to be excluded since one always aimed at avoiding as carefully as possible that even traces of grease should remain behind in the high-vacuum chamber.

The present invention affords for the irst time a practical `solutionof the problem of utilising ball-bearings for journalling X-ray anodes of the heavy type.

During operation of an X-ray tube the anode, more particularly the rotary anode adapted to be submitted to a high load, is thermally loaded, which results in an increase of the temperature of the bearing members `from which, as from any other part of the anode, gases may be liberated if previously they have not been removed therefrom. The ball-bearings cannot sustain the high temperature required for the careful removal of occluded gases and besides, during operation of the tube, they are very sensitive to heating.

In an X-ray tube according to the invention, the above diliculties are overcome by making the thermal conductivity of the mechanical connection between that part of the anodelon which during operation the heat is developed and the bearing parts so small that from the thermally loaded parts towards the bearings conduction of heat does practically not occur, and by connecting the bearing parts in a satisfactorily heat-conducting manner to members which can be cooled during operation. A heavy anode, i. e. an anode wherein the metal exposed to the electron bombardment is connected in' a satisfactorily heatconducting manner to a metal mass to which the heat is transferred and in which it can be dispersed at once, has, in contrast with the disadvantage it offers to rotatable journalling due to its weight and size, the advantage that its tempe'rature remains several hundreds of degrees centigrade' below the temperature which is ac-l quired during operation by a light anode, i. e. an

anode of refractory metal having no :appreciable Y,

thermal capacity. For this reason the thermal insulation of the bearing parts, for which purpose use may be made of metal of poor heat-conductivity as well as of heat-proof insulating material,

with respect to the heated rotary body is less difcult with a heavy anode than would be the case with a light anode. y

If these conditions are fullled, it is possible to provide the ball-bearings of the rotary anode with a lubricant which has so low a vapour pressure that it has no appreciable harmful influence on the high vacuum in the tube. According to the invention, the known so-called cock grease, which is used by persons in the physical art for the sealing of exhausted vessels, more particularly of cocks, but hitherto only for this purpose, is utilised as a lubricant for the ball-bearings.

According to the invention, the suspension of the comparatively heavy `rotary body may be effected by means-of a thin-walled tube which is secured between the front of the rotary bearing natively extend from the front of the rotary bearing member backwards to the anode end of the tube and be secured'to the rotary anode body at a point remote from the anode target.

A suitable material for the intermediate member is ferro-chromium, which'possesses both a great mechanical rigidity and a slight heat conductivity.

The thin-walled metallic tube should preferably be protected against electronv bombardment since, on account of the small thickness lof itswall, it might be heated to an admissibly high temperature even by a stream of electrons of comparatively low intensity.

It is furthermore advantageous if the ballbearings are provided at points which communicate with the discharge chamber proper only by lmeans of narrow channels having cool walls.

a getter which absorbs any gases or vapour set the electric Iield of the tube according to the U. S. Patent #2,134,348. The tube may alternatively be provided with a device which produces a getter at an arbitrary instant after the exhausted vessel has been sealed off.

The invention will be explained more fully with reference to `the accompanying drawing which represents, by way of example, two forms of construction of tubes provided with rotary anodes according to the invention.

Fig. 1 represents a longitudinal section of a tube having an intermediate member which extends from the bearing members towards the cathode.

Fig. 2 represents an anode construction in which the intermediate member extends backwardly from the foremost part of the bearing.

In Fig. 1, reference numeral I denotes a metallic portion of the wall of a tube. To the edges yof portion I are sealed glass portions 2 and 3. The portion 3 has a re-entrant portion 4 which supports a. cathode construction 5. The portion 2 also has a short re-entrant portion 6 the edge of which is sealed to a metallic body 1. In this body there are provided ball-bearings 9 and Il! in which a rotary shaft I I is journalled.

In the interstices 2| between the balls there is a small amount of cock grease which has a very low vapour pressure so that upon being heated to say 200 C. it does not injuriously affect to an appreciable extent the high vacuum in the tube.

The foremost end of the shaft II is provided with a disc I 2 which carries a thin-walled metallic tube I3. This tube constitutes an intermediate member between the rotary bearing member and the rotary anode body I4. This body consists of a comparatively heavy piece of copper provided with a backwardly extending cylindrical extension I5 which acts in a manner known per se as the rotor of an electric motor. The glass portion 2 is surrounded by an electromagnetic stator (not shown in the drawing) which sets up the magnetic rotating field that causes the anode to rotate. The front of the body I4 has secured to it a tungsten disc I6 which is turned towards the cathode 5, said disc being in satisfactory heat-conducting connection with the body I4. The heat produced in this disc during operation of the tube is directly distributed over the mass of the body I4 whose temperature may increase in the case of prolonged operation, for example to 500 C. It cannot bring about, however, an appreciable increase in the temperature of the ballbearings and this for the following reasons.

The thin-walled metallic tube I3, which preferably consists of ferro-chromium and has a wall thickness, for example, of 0.5 mm., transfers, due to its slight thermal conductivity, only a small amount of heat to the ball-bearings. Ii the bearings were mounted so as to be thermally insulated, yet even a very small supply of heat could increase their temperature in the long run to an appreciable extent, which might result in that the lubricant decomposes or vaporizes and that the ball-bearings, which are only partly deprived of occluded gases, become jammed. The ball-bearings are, however, in heat-conducting connection with the member 1 which can be cooled during operation and which takes up at once all the heat which is still transferred by the metal tube I3 to the bearings. The ball-bearings consequently remain cold and the grease as well as the bearings themselves do not give rise to inconveniences.

The heat radiated by the cylindrical portion I5 of the rotary anode body is partly taken up by the member 'I which permanently be cooled, so to speak, may be thermally grounded during operation (if required, by additional means, for example by bringing it into contact with a liquid or gaseous cooling medium or a solid cooling body).

With the anode according to Fig. 2 the reentrant portion 6 has sealed to it a metallic plate E, which carries a cylindrical casing I9 for the ball-bearings 9 and ID which are provided with grease. The metallic plate 8 and therefore the casing I9 with the ball-bearings may be kept at a low temperature during operation by artificial cooling. The disc I2 mounted at the end of the shaft II has secured to it a metal tube II which has a thickness, for example, of 0.5 mm., said tube extending backwardly and being secured at its other end to the hindmost edge of the cylindrical portion I5 of the anode. This tube is preferably made of ferro-chromium since this metal has a great mechanical rigidity and a slight thermal conductivity. Besides, the metal tube I'I forms a screen which prevents heat from being radiated from the cylinder I5 towards the bearing members.

The construction of the tube as shown in Fig. 1 is suited for taking radiographs and for fluoroscopy. The anode according to Fig. 2 does not surround, at a slight distance, as in Fig. 1, a body adapted to take up the heat, and is less suitable for prolonged operation.

Both practical examples that are shown are characterised in that the points where the ballbearings are provided communicate with the discharge space proper only by narrow channels having cool walls. If grease particles were to be liberated from the ball-bearings, it is yet impossible ior them directly to reach the discharge path and to come into contact with the hot parts of the tube. With the tube according to Fig. 1 they would have to pass to that end through the long and narrow passage between the bodies I5 and 1. 'Ihe grease molecules are retained, however, by the cool member l. In the form of construction according to Fig. 2 any grease particles which may be set free from the ball-bearings pass along the cool cylinder I9 since they can escape to the discharge chamber only through the narrow passage between this cylinder and the metal tube I1, which chamber they will, however, not easily reach.

The aperture in the front-side of the anode body I4, which serves to attach the metal tubes I3 or by means of a suitable tool to the disc I2, is closed by a screw I8 so that the electrons are prevented from entering into the chamber enclosed by the body I4 and from heating the thinwalled metal tube I3 or I'I.

In the tube according to Fig. 1 the cathode construction consists of a tube which acts as a concentrating device and from which a portion is separated by a partition 22. This portion contains a heating helix 23 and a getter receptacle, for example a copper tube filled with barium. The heating body may be heated by a current supplied thereto by wires 24 and 25 in order to vaporize the getter. The condensed vapour deposits on the inner wall of the chamber separated by the partition 22. This getter chamber communicates by means of an aperture 26 with the remainder of the high-vacuum chamber so that gases or vapours which may escape, for example, from the grease in the ball-bearings or from the bearings themselves and which would deteriorate the vacuum, are at once absorbed and rendered harmless by the getter in the chamber below the disc 22.

Charge carriers which are accelerated in the electric eld of the tube cannot strike, however, the getter deposit. This is very important since otherwise if such particles would im'pinge thereon the absorbed gases could be expelled again from the getter.

The current supply Wire 2'! as well as the Wire 24 serve for the supply of heating current to the incandescent body 28 of the cathode.

The concentrating device of the cathode comprises by a screen 29 which surrounds the foremost anode portion an'd prevents electrons from passing from .the discharge chamber into the remainder of the evacuated space, harmful discharges which might be produced by such leakage electrons being thus avoided.

The screen 29 and the metallic wall portion I have windows 30 and 3| for the passage of the beam of useful rays. The Window 30 is closed by a beryllium disc Whereas the metallic portion I,

`which consists of ferro-chromium, has a glass window sealed to it.

Notwithstanding the use of grease in a highvacuum tube, which use would almost appear absurd to the man in the art, the means utilised in accordance with the invention afford a device which is very reliable in operation. The fact that the anode body journalled on balls can easily rotate renders it possi-ble to operate the tube in any position and to utilise a stator of very small power and of small size.

What We claim is:

1. An X-ray tube comprising an envelope, a cathode, a rotatable anode of the heavy type, a member adapted to be cooled lduring operation of the tube, a ball-bearing supported from said member, a second member of low heat-conductivity supporting said anode from said bearing to prevent heating of the ba'll-bearing and a lubricant of the type of cock grease on said ballbearing.

2. An X-ray tube comprising an envelope, a cathode, a rotatable anode of the heavy type, a ball-bearing, supporting means of good heat-conductivity for said bearing, a thin-walled tube of a metal of low heat-conductivity supporting said anode from said bearing to prevent the transfer of heat from the anode to the Iball-bearing, and means to protect said tube from electron bombardment during the operation of the X-ray tube.

3. An X-ray tube comprising an envelope, a cathode, a ball-bearing, a rotatable anode of the heavy type, means of poor heat conductivity supporting said anode from said ball-bearing, a lubricant on said ball-bearing, and gettering means Within said envelope for removing any gases or vapors liberated by the ball-bearing and lubricant.

4. An X-ray tube comprising an envelope, a cathode, a rotatable anode, a ball-bearing supported from said envelope, means of low heatconductivity supporting said anode from said bearing to Iprevent heating of the ball-bearing, and a lubricant on said ball-bearing, said lubricant having a low vapor pressure.

JACOB HARMANNUS VAN DER TUUK. ALBERT BOUWERS. 

